Guard against incorrect use of compose.set_mode

(imported from commit 294441f2b9107ce2488866764c3d30c35133a14a)

zephyr/static/js/compose.js

@@ -153,6 +153,9 @@ exports.clear = function () {
 // Set the mode of a compose already in progress.
 // Does not clear the input fields.
 exports.set_mode = function (mode) {
+    if (!is_composing_message)
+        return;
+
     if (mode === 'private') {
         show('private', $("#private_message_recipient"));
         is_composing_message = "private";